Book - School of Science and Technology

640 Automatic controls and building management systemsControlvalvePumpBoilerplantTTOutsidetemperature sensorOptimum startcontrollerInsidetemperature sensorFigure 22.23 Optimum-start control systemIn use Unoccupied In use20Room temperature ºC10Decay curvesvary withoutsidetemperatureMinimum to preventcondensationMaximumpre-heatLevel & periodof pre-heatvaried01200 2400 1200TimeFigure 22.24 Characteristics of optimum-start controland also the rate at which the heating system can heat up the space. Generally, wetheating systems have a slow response compared with air based systems, and in consequenceneed to be switched `on' earlier. In hotter climates optimum start may be appliedto space pre-cooling, but this facility is generally less applicable in the climate of theBritish Isles.Regardless of the method employed for system start-up, separate frost protection is anessential feature for all systems in order to protect the conditioned spaces from condensationrisk (normally 10 C is the lowest acceptable temperature) and to protect plantcomponents from freezing. As moving water is less prone to freezing than when static,it is normal where parts of the system are subject to outside temperatures, that the firststage of a frost protection strategy, is to switch on the circulating pumps.